Catalytic cracking heat exchange process



NOV l5, W N. MCCURDY JR CATALYTIC CRACKING HEAT EXCHANGE PROCESS FiledAug. l, 1950 Clbborrle United States Patent O l CATALYTIC CRACKING HEATEXCHANGE PROCESS William N. McCurdy, Jr., Linden, N. J., assignor toEsso Research and Engineering Company, a corporation of `DelawareApplication August 1, 1950, Serial No. 176,979 2 Claims. (Cl. 196-52)This invention generally concerns a catalyticcracking operation in whicha gas oil fraction of a petroleum crude oil is cracked to yield lowerboiling constituents. In particular the present invention concerns thespecific problem of operating heat exchangers in conjunction with afluidized catalytic cracking operation.

The principal` object of this invention is -to overcome certain problemsof heat exchanger fouling ordinarily encountered in processing thestreams connected with a liuidized catalytic cracking operation. Theparticular heat exchanger referred to is a heat exchanger conventionallyused to preheat the oil fed to the catalytic cracking reactor. Thus, itis necessary to preheat the oil feed to a temperature of 500 to 700 F.to secure the `desired temperature conditions before injection of theoil into the catalytic reaction vessel. A large portion of this heatingrequirement can be met by passing the oil feed in heat exchangerelationship with certain streams of the catalytic cracking system.However, it has been found that relatively rapid fouling of the heatexchanger in which this is carried out occurs. The fouling isapparentlydue to deposition on the heat exchange coils of gummy material;presumably caused by polymerization of constituents usually occurring inthe presence of oxygen dissolved in the oil feed, or possibly duetocarbonization of portions of the feed. It appears that these foulingconstituents deposit `on heat exchange tubes when the oil feed has beenraised to temperatures in the range of 250 to 400 F.

The fouling of these heat exchange tubes is clearly disadvantageous.First, it results in the lowering of heat exchange co-eftcients so thateven after a very short period of use, new or cleaned heat exchangetubes will lose some portion of their heat transfer capabilities. Theresult is `that heat exchange to the oil feed will become ineicient andtemperature requirements in the entire catalytic cracking system maybecome maladjusted. At times the failure to preheat the oil feedsufficiently has been responsible for directly affecting the throughputat which a catalytic cracking unit could be operated. To keep thisdiiiculty within limits it consequently becomes necessary to frequentlyclean the heat exchange surfaces.

Heretofore the cleaning of heat exchangers employed in the indicatedService has been a diiiicult and expensive problem. While the use ofsolvents has been attempted, no solvent has been found which is suitablefor adequately removing the gummy constituents so as to properly cleanthe heat exchange surfaces. It has consequently been the generalpractice to partially, or completely dismantle the heat exchangeequipment to permit mechanical cleaning of the tube surfaces. As theheat exchangers are generally arranged in the form of bundles of a largenumber of tubes, it is difficult, or impossible to adequately clean manyof the tubes of the bundle except at their points of tangency with acleaning bar which may be inserted through the bun- 21,723,948! PatentedNov. 15, 1955 dle. Again, when the tube bundle has become slightlydeformed it is often impossible to introduce a cleaning bar through thebundle. Similarly, it has been found that Sandblasting is an expensiveand time-consuming method to clean the tube bundles, as the sand doesnot penetrate to the inner tubes unless great care is used, but simplycleans the outer tubes of the bundle.

For these reasons, therefore,` adequate means to sm` ply and effectivelyclean the tubes of a heat exchanger associated with a catalytic crackingunit has been aV serious operational problem. As will be disclosed, theprocess of this invention adequately solves this problem in anadvantageous manner.

The specific nature of the problem involved, and the manner in which theprocess of this invention solves this problem may be appreciated byreference to the accompanying drawing, and the following description.The drawing to which the description relates, diagrammaticallyillustrates one ow plan employed in catalytic cracking systems.

Referring now to the drawing, the numeral 1 identifies a catalyticcracking reactor in which the cracking operation may be conducted. Thenumeral 2 represents the associated catalyst regenerator and theremaining processing units shown in the drawing identify the slurryequipment and associated apparatus required. The catalytic crackingprocess conducted in reactor l is of the nature identified as aliuidized catalytic cracking operation. The term fluidized identitiesthe manner in which the solid catalyst particles are maintained as adense boiling bed by passing gas through the particles to maintain ahigh degree of turbulence. Thus, if oil feed which already is vaporized,or will become vaporized upon contact with the hot catalyst, dependingon the oil temperature, is introduced through line 3 into the lowerportion of reactor 1 together with solid particles of catalyst, bycritically adjusting the rate of ow of the oil feed, and the proportionof solid catalyst particles, it is possible to maintain the catalyst particles in reactor 1 in a fluidized state. The catalyst particles willunder these conditions assume the characteristics of a liquid so that anupper level 4 may be maintained in the reactor. The catalyst particlesbelow the level 4 will beA in the form of a dense bed of turbulentparticles visually appearing as a boiling liquid-like mass. The gaseousoil feed will pass upwardly through this mass of catalyst particles tobe removed from the top of the reactor through line 5.

As the process described is a continuous process, catalyst and oil feedare continuously introduced to the lower portion of reactor 1 asdescribed, while cracked products are removed at the top of the reactorthrough line 5 as described, and catalyst is continuously removed fromthe bottom of reactor 1 through draw-o 6. The catalyst Withdrawn throughdraw-off 6 may be said to be spent catalyst, having been retained in thereactor 1 for sufficient periods of time to have become fouled withcarbon and gummy material. This catalyst may be regenerated by theprocess of burning the fouling constituents from the catalyst. For thispurpose air may be passed through line 7 to carry the spent catalystupwardly through line 8 to regeneration zone 2. In zone 2, as in reactor1, the catalyst is maintained in a fluidized condition by the air. Bysuitably controlling the air to catalyst ratio, and the temperature ofthe system, it is possible to oxidize or burn the catalyst impurities soas to regenerate the catalyst. Regenerated catalyst may then beWithdrawn from the regenerator through standpipe 9 for recycle toreactor 1 as formerly described.

The gas velocities employed to secure the desired fluidization of thecatalyst in reactor 1 and regenerator 2 are a slurry, and is generallyidentified as slurry oil.

generally in the range of aboutV l to 3 feet per second. While thedensity of the catalyst particles in the gas under uidized conditionswill depend upon the catalyst particle size, and the particulargasvelocity employed, in general the density of the fluidized bed will beabout 10 to 25 pounds per cubicV foot. Gas and catalyst ow rates areadjusted to-secure this density. Reactor 1 is maintained at temperaturesin the range of 900 to 1000 F. while regenerator 2 is maintained attemperatures of about 1050 to 1150 F. The catalyst employed may beselected from vany of the known cracking catalysts, such as aluminasupported on Group 6 metals. Similarly, the catalyst may be cobalt,nickel, iron or compounds of Group 6 oxides, with nickel, cobalt oriron. VThe catalyst is preferably in powdered form having a size ofabout 200 to 400 mesh, although particles outside this range may be em-Y ployed.

Insofar as the cracking process heretofore generally identified is wellknown to the art no further description of these elements of the presentinvention will be given. To understand the process of this invention,the description will now be given of the manner in which the productstream from reactor 1 is processed, and the manner in which the oil feedpassing through line 3 is preheated.

Referring first to the processing of the cracked product stream of line5, it may be noted that this stream will consist principally of crackedconstituents originally present in the oil feed. As conversion is not100%, the stream of line 5 will also contain some portion of theuncracked original constituents of the oil feed. ln addition, somecatalyst particles will be carried from reactorV 1 in line 5.

In this connection it may be noted that a cyclone separator or severalstages of cyclone separators may be employed to minimize catalyst carryover in line 5. However, it is impossible to avoid some catalystentrainment. For this reason the stream of line 5 will contain about 2%of the catalyst introduced to the bottom of reactor 1. The stream ofcracked products from the reactor 1 is then conducted through line 5 tothe fractionator 10. As illustrated, the fractionator may consist of aconventional type of fractionator employing a bottom pump aroundcircuit. The fractionator may contain any desired type of fractionatingplates, or packing material, or as illustrated, the upper part offractionator 10 may contain fractionating plates 11, while the lowerpart of fractionator 10 may contain the disc and donut packingdiagrammatically illustrated and-designated by numeral 12. The productsof line 5 are introduced to the bottom of fractionator 1t), and thefractionated constituents are then removed from fractionator 10 throughoverhead line 13, and side stream withdrawal lines 14 and 15. Theheavier boiling constituents existing in the material introduced to thefractionator are withdrawn from the bottom of fractionator 16, throughline 16. These heavier boiling constituents will contain catalystcarried over from reactor 1. As this catalyst has now been Vconcentratedin the lower boiling fractions, the catalyst concentration will be about0.2 pound per gallon to about l pound per gallon, although normally thecatalyst concentration is about 0.4 pound per gallon. This concentrationof catalyst in the heavy constituents of line 16 provides a heavy oilappearing as The stream of slurry oil withdrawn through line 16 is thensubdivided to pass in part through the waste heat boiler 17 and the feedpreheater 18. The portion of the slurry oil passed through the wasteheat boiler 17 is passed in heat exchange relationship with water so asto remove heat from the slurry oil and simultaneously produce usefulsteam. The cooled slurry oil is then re-introduced to fractionator 10through line 19. The portion of the slurry oil pumped through the feedpreheater 18 is similarly passed in heat exchange relationship with oilfeed so as to preheat the oil feed. This is generally conducted, asshown by the solid lines, by passing theslurry oil of line 16 throughthe coils 20 of the preheater 18. The oil feed pumped through line 1 iscirculated around the outer portion of the coils 20 for removal from theheat exchanger through line 3. The slurry oil withdrawn fromfractionator 10 has a temperature of above 500 F., or preferably about600 or 650 F., and in passing through preheater 18 is generally cooledto about 450 F. This cooling of the slurry oil is effective to heat-oilfeed, by heat exchange, to a temperature of about 400 F; and preferablyto above 500 F. As formerly indicated, operation of this feed preheaterresults in the deposition of fouling materials on the coils 2,0. In atypical commercial installation, ithas been found that heat exchangecoefficients can drop fromabout 50 to 30 in a period of l5 days, as aresult of this fouling. Ordinarily this necessitates cleaning of coils2t) after about l5 days of operation.

in accordance with this invention, the undesired drop in heat transferco-efficients, and the necessity for removiug the heat exchanger Yfromthe system for cleaning, may be Vavoided by periodically altering theflow Q the slurry oil and oil feed through the heat exchanger. TheneCessary alteration in heat exchange operation is indicated by thedashed. lines in the drawing. Thus. Periodically, slurry oil is cutofffrom passage through coils 20 and is passedl through line 22 so astopess around, rather than through Coil v20. Theslurry oil may theo beremoved from the heet. exchanger through line 23. Simultaneously oilfeed through the shell of the preheater is discontinued, and the oil ispassed-through line 24 for introduction directly into coil 20 and yforremoval from coil 20 through line 25. For simplicity, the valvesrequired in the lines associated with the preheater have not beenillustrated. `It is apparent that by these changes the normal ow ofslurry oil through coils 20 and oil feed around coils 20 is reversed. Inthe reversed condition, oil feed is passed through coils 20 while slurryoil is passed through the shell around coils 20. It has been found thatby periodically reversing the slurry oil and oil feed flows in thismanner, fouling of heatvexchanger 2,0 may be substantially eliminated.At the same time the undesired drop in the heat transfer co-eflicientsof the heat exchanger is substantially `eliminated.

As a specific example of a process employing this invention, acommercial catalytic cracking operation was conducted embodying thisinvention. During the test period slurry oil was introduced to the heatexchanger at a temperature of about 600 F. and was withdrawn at atemperature of about 45.0 F- Oil feed, was .introduced to the heatexchanger at a temperature of about 250 F. and was withdrawn at atemperature of about 450 F. About 60,00() gallons per hour of slurry oiland about 35,000 gallons per hour of oil feed were pumped through theheat exchanger. The Catalyst coutent of the slurry oil was 0.4 lb. ofcatalyst per gallon o f oil. At the beginning of the test period, theoutside of coils 20 were fouled. Upon interchange of the two fiuids for48 hours, the slurry cleaned the outside of the tubes. This was provenbecause when the uids were again interchanged to their original flowcourses, it was found that the overall heat transfer coeicients oftheheat exchanger were improved an average of 15.7 B. t. u.,/ hr./ sq.foot/ F. and the oil feed outlet temperature had been increased anaverage of 36V F. as a result of this operation. Furthermore, it wasfound that the heat transfer coefficients were maintained equal to thosesecured in fresh mechanically cleaned heat exchange bundles.

As described, therefore, the process of this invention specificallyConcerns the operation of the oil feed preheater associated with acatalytic cracking operation. The oil feed preheater passes an oil feedin heat exchange relationship with hot slurry oil from thecatalyticcracking operation. By periodically reversing the paths of flow of theoil feed, and slurry oil through the heat exchanger, materialoperational advantages are securedr as described. lt is apparent thatthe frequency of how reversal in this manner may be chosenas desired.Experience has shown that flow reversal about every 15 days isdesirable, and that this flow reversal must be maintained for about 48hours to secure effective cleaning by the slurry of the outside of thetubes. While the invention has been described with regard to aparticular type of fluidized cracking operation, it is apparent that theinvention is equally applicable to other embodiments of tluidizedcracking processes. For example, while the process described toexemplify the present invention consists of what has been described as adownflow catalytic cracking operation, this invention is equallyapplicable to upllow cracking operations.

What is claimed is:

1. A catalytic cracking operation which comprises heating a gas oil feedto a temperature in the range of 400 to 650 F. in a preheating zone,catalytically cracking said heated gas oil in a fluid catalyst crackingzone, passing the cracked products from said cracking zone together withentraincd solid catalyst to a fractionation zone, separating from thecracked products a hot high boiling hydrocarbon oil slurry fractioncontaining about 0.2 to 1% of solid catalyst particles per gallon ofoil, passing the said hot high boiling hydrocarbon slurry fraction fromsaid fractionation zone in indirect heat exchange relationship with thesaid gas oil feed stream in said preheating zone, and periodicallyreversing the flow of the feed gas oil and high boiling hydrocarbonslurry fraction while substantially maintaining flow of said liquids soas to bring the gas oil feed and high boiling slurry fraction onopposite sides of the heat exchange surfaces of the said preheating zoneand maintaining such reversed ow for an extended period of time wherebythe solid particles in the slurry remove fouling material from saidpreheating zone deposited from said gas oil feed and to improve the heattransfer coefficient between the two oil streams.

2. A catalytic cracking operation which comprises heating a gas oil feedto a temperature in the range of 400 to 650 F. in a preheating zone,catalytically cracking said heated gas oil in a fluid catalyst crackingzone, pass ing the cracked products from said cracking zone to# getherwith entrained solid catalyst to a fractionation zone, and separatingfrom the cracked product a hot high boiling hydrocarbon oil slurryfraction containing about 0.2 to 1% of solid catalyst particles pergallon of oil, passing the said hot high boiling hydrocarbon slurryfraction at a temperature of about 650 Fn from said fractionation zonein indirect heat exchanger relationship with the said gas oil feedstream in said preheating zone, and periodically reversing the flow ofthe feed gas oil and hot high boiling hydrocarbon slurry fraction whilesubstantially maintaining ow of said liquids so as to bring the gas oilfeed and high boiling slurry fraction on opposite sides of the heatexchange surfaces of the said preheating zone and maintaining suchreversed flow for about 48 hours whereby the solid particles in theslurry remove fouling material from said preheating zone deposited fromsaid gas oil feed and to improve the heat transfer coeflcient betweenthe two oil streams.

References Cited in the flc of this patent UNITED STATES PATENTS2,210,257 Pyzel et al. Aug. 6, 1940 2,353,399 Herthel July l1, 19442,423,833 Hirsch July 15, 1947 2,493,494 Martin Jan. 3, 1950 2,576,843Lockman Nov. 27, 1951

1. A CATALYTIC CRACKING OPERATION WHICH COMPRISES HEATING A GAS OIL FEEDTO A TEMPERATURE IN THE RANGE OF 400* TO 650*F. IN A PREHEATING ZONE,CATALYTICALLY CRACKING SAID HEATED GAS OIL IN A FLUID CATALYST CRACKINGZONE, PASSING THE CRACKED PRODUCTS FROM SAID CATALYST CRACKING ZONETOGETHER WITH ENTRAINED SOLID CATALYST TO A FRACTIONATION ZONE,SEPARATING FROM THE CRACKED PRODUCTS A HOT HIGH BOILING HYDROCARBON OILSLURRY FRACTION CONTAINING ABOUT 0.2 TO 1% OF SOLID CATALYST PARTICLESPER GALLON OF OIL, PASSING THE SAID HOT HIGH BOILING HYDROCARBON SLURRYFRACTION FROM SAID FRACTIONATION ZONE IN INDIRECT HEAT EXCHANGERELATIONSHIP WITH THE SAID GAS OIL FEED STREAM IN